608 Subsurface Geologic Methods 



By properly evaluating the many variables involved, a better under- 

 standing of the limitations of the magnetic method of orienting cores is 

 possible. Further, a knowledge of these variables should be of assistance 

 in interpreting the orientation results. 



Because of the magnitude of the variables it is obvious that too much 

 weight should not be attached to any individual determination. Instead 

 a statistical approach seems essential in that an average value of a number 

 of determinations is of much greater significance than any particular de- 

 termination. Likewise, an average direction signifying a dip in a certain 

 quadrant, as NE, NW, SE, SW, should be of the right order of magni- 

 tude. Thus, whenever a magnetic dip is to be determined four or five 

 pieces of the core should be submitted. Each piece should be at least two 

 to three inches in length. By orienting each piece and averaging the re- 

 sults an average vector specifying the direction and amount of dip of the 

 cored interval is obtained. The individual determinations may be made to 

 the degree, and perhaps the average as well, as long as it is understood 

 that the average vector is to be interpreted as specifying a general and 

 not a precise direction. 



Magnetic orientation is by no means a "cure all." In fact, determina- 

 tions should be carefully weighed in regard to the magnetic variables that 

 were operative during the history of the sediment. Next, the weighed 

 opinion should be considered in the light of all other available geologic 

 and geophysical evidence. Only in this manner can the results of magnetic 

 determinations of strike and dip be used to advantage. 



Practical Applications 



Normally, cores are oriented for the purpose of increasing the sub- 

 surface control and thus aid in the drilling program of the well as well 

 as in the development of a field. The recognition of subsurface faults is 

 facilitated by an oriented-core program. The direction of dip found mag- 

 netically may show that the well has cut a fault. The beds above the fault 

 may dip in one direction, while those below, perhaps of the same forma- 

 tion, may be found to dip an equal amount but in an entirely different di- 

 rection. In this case, without the information as to the sudden change in 

 the direction of dip with increasing depth, there may be little reason to 

 suspect the presence of a fault, the simplest explanation being merely an 

 assumption that there existed an entirely conformable relationship, perhaps 

 with some thickening and no faulting. The passage of a well from one 

 limb of a steep fold to the other may be similarly detected by oriented 

 cores. 



In addition to such practical problems, others, such as those involv- 

 ing rates of sedimentation and short-range correlations, may prove worthy 

 of serious consideration. 



